1. Field of the Invention
This invention relates to television systems, and more particularly to scrambling and unscrambling (or encoding and decoding) video signals by selectively inverting these signals.
2. Description of the Prior Art
The prior art discloses primarily four techniques which are used to scramble video signals so that the reception of the signals results in an unintelligible picture on a television receiver screen. The first way that such scrambling may be accomplished is to change the frequency of the transmitter signal. A second way is to modulate the video portion of the signal with another video frequency electronic signal. A third method of scrambling video signals is to modify the real time position of the video. Finally, the fourth method is to modify the horizontal or vertical synchronization portions of the signal (sync signals) which will result in the receiver losing vertical or horizontal hold.
An example of the fourth method mentioned in the preceding paragraph is found in U.S. Pat. No. 3,813,482. The transmission of video signals is scrambled by repetitively depressing to a blanking level a plurality of sync signals, preferably the vertical sync signals, which produces a shifting and rolling picture.
Another means of varying the synchronizing information is disclosed in U.S. Pat. No. 3,801,732. This patent discloses the reversing of polarity of both the video and synchronizing information.
In U.S. Pat. No. 3,824,332, reference pulses of opposite polarity to the horizontal sync pulses are added to the composite television signal preceding each horizontal sync pulse. Video portions of the signal are inverted for randomly selected fields, after a code burst is added to the signal to indicate that the field immediately following the coding bursts is inverted.
Other examples of alternate scrambling and descrambling systems have been developed in the last several years in response to the possibility of producing pay or subscription television both by means of cable transmission and over-the-air transmission of signals.
One of the prime considerations in such scrambling and unscrambling systems is that the internal circuitry of a television set must not be altered by the unscrambling apparatus. That is, any unscrambling apparatus at a receiver must be independent from the internal workings of the television set itself, but must be able to be connected to the set in a manner that will accomplish the purposes of the pay or subscription television.
Of primary consideration in any pay or subscription television system which is added to an existing television set, such as a receiver in a home, or to a transmitter at a station, is the cost of the apparatus, in terms of not only manufacturing costs, but also in terms of installation costs and maintenance costs. Another consideration is with respect to the end product or picture which will be displayed on the receiving set resulting from the use of the apparatus. In other words, a factor to be considered is whether or not, or if so, to what extent a scrambling and unscrambling system will have an effect on the quality of the signals ultimately used to provide a picture on the picture tube. Obviously, a scrambling and descrambling system which is expensive to fabricate, difficult and therefore expensive to install, and difficult to maintain will have problems in terms of outright economic obstacles. The derogation of the signal resulting from the installation of such scrambling and unscrambling system is also of importance. No matter how acceptable in terms of monetary or economic considerations a scrambling and descrambling system may be, if the use of such system results in a poor quality picture, the entire system may be unfeasible.
Perhaps the most necessary ingredient to make subscription television a success is to have the ability to change scrambling codes easily. Individual codes for each television program is the best and most ideal situation. This would permit separate changes or billings for individual programs. The economic advantages of this type of system are obvious. Prior art apparatus is not readily adaptable to assign individual scrambling codes for a particular television program. The apparatus of the present invention has the advantage of being able to do so.
No new frequencies are required for the present apparatus. Accordingly, the system can be used with presently established broadcast stations.
In some of the prior art apparatus, it is difficult to change the scrambling codes once a system is installed in a set. However, in the present apparatus, scrambling codes may be easily changed. This inhibits possible selling of "black-market" decoders.
Briefly, there are some disadvantages of the prior art methods of scrambling and unscrambling as exemplified by the methods and apparatus discussed above, and particularly in some of the noted patents illustrative of the prior art as discussed above. The suppressing of synchronization (sync) pulses is rather expensive and there is a limited number of codes which can be employed under such circumstances. The shifting of real time is also rather expensive. A limited number of codes can be employed when a television signal is modulated with another video signal. If coding and decoding apparatus requires the changing of transmitter frequencies, there are immediate problems with the Federal Communications System (FCC). These economic (and other) problems are not found in the present invention because none of the factors are present. Instead, a relatively simple method is employed which is compatible with existing situations requiring no major changes or alterations.
The apparatus of the present invention comprises a scrambling and unscrambling (encoding and decoding) method and apparatus substantially different from the prior art as discussed above. The prior art apparatus and methods include the reconstituting or decoding of the scrambled signal prior to the television receiver antenna. This situation is expensive because of the required use of radio frequencies, and tuned circuits, all of which are expensive and relatively temperamental. The present apparatus does not reconstitute the television signal prior to the antenna, but merely prior to the television picture tube.
The present apparatus does not change the broadcast band or sync pulses, and no change is made in any existing composite TV signals, such as times, blanking pulses, or sync pulses. No new sync pulses or information pulses are added.
There are no tuned circuits in the present apparatus and no work is accomplished in the high frequency range. Moreover, the apparatus lends itself to the use of integrated circuits. A substantial economic advantage results. In addition, the prior art does not lend itself for inclusion into newly manufactured TV sets. The present decoding apparatus would result in a very minor design change and would be included in newly manufactured TV sets for a slight increase in the cost of the set.
As a separate unit, the apparatus included in the present system is relatively inexpensive to manufacture, to install, and to maintain, and the resulting picture is relatively unaffected. In some cases, a color picture may be enhanced by the use of the apparatus.